Polyurethanes are a well-known type of adhesive. They contain precursor materials that cure in place to form an adhesive layer. Polyurethane adhesives come in one-part and two-part types. One-part types generally exhibit a moisture cure or a heat-activated cure. Two-part types consist of a resin component that includes one or more polyisocyanate compounds, and a curative component that includes one or more polyols. When the two components mixed, the polyisocyanates and polyols react to form a cured polyurethane adhesive. A polyurethane adhesive can be formulated to cure at room temperature or upon exposure to certain conditions, an example of which is an elevated temperature. As the adhesive cures, it can form a strong adhesive bond to many types of substrates.
Two-part polyurethane adhesives are usually formulated with a small amount of an amine compound that has primary and/or secondary amino groups. Amino groups react very rapidly with isocyanates. This rapid reaction results in a fast initial viscosity increase without gelling the material. The initial increase in viscosity makes the adhesive more resistant to sagging or running off before it can be cured.
The presence of the amine compound tends to shorten open time and to adversely affect storage stability. For those reasons, it is desirable in some cases to eliminate amine compounds from the adhesive composition, while maintaining a suitable open time and thereafter obtaining a fast cure. The “open time” of a two-part adhesive refers to the amount of time after the two components are mixed that the adhesive remains flowable and capable of bonding to a substrate.
One way of obtaining both a long open time and a fast cure is by formulating the adhesive to have a heat-activated cure. Such an adhesive cures slowly at ambient temperature, thereby allowing the adhesive to be applied and the substrates positioned while the adhesive remains flowable. The resulting assembly is then heated to an elevated temperature at which rapid curing takes place.
Heating using conventional curing ovens tends to be slow and expensive, and may not be well-suited for large or complex assemblies. To lower costs and speed the cure, infrared heating methods have been developed. These methods permit the substrate/adhesive assembly to be brought more rapidly to the curing temperature than with convection ovens.
Infrared heating methods can be used to partially cure the adhesive to develop enough initial bond strength to allow the adhered assembly to be handled in downstream manufacturing operations, while allowing full cure to develop later. This approach can speed production processes, as it is not necessary to wait for the adhesive to fully cure before subsequent manufacturing steps can take place.
Infrared heating has a further advantage in that it can be targeted, so only specific locations of the assembly are heated. This allows for spot-curing, i.e., curing only predetermined portions of the adhesive. Strong localized adhesive bonds form where the localized heat is applied. Even though much of the adhesive remains uncured or only partially cured, these localized bonds provide enough strength to the assembly that it can be manipulated. This process saves time, because localized heating can be accomplished more rapidly than curing the entire assembly, and saves energy as only a portion of the assembly needs to be heated. The subsequent full cure can take place at ambient temperature, or if an elevated temperature cure is needed, this full curing step sometimes can be combined with another manufacturing step, such as a paint curing step, to save costs and speed production rates.
What is wanted is a two-component adhesive that demonstrates a prolonged open time at room temperature, but nonetheless is capable of curing even at room temperature. The adhesive preferably develops bond strength rapidly when exposed to infrared heating.